During normal mitogenesis, balanced action occurs between 1) growth factor proteins that stimulate and 2) p53 and other suppressor proteins that inhibit the cell cycle. For example, fully phosphorylated p53 binds to the promotor for Waf 1 and the resulting gene product prevents the phosphorylation of down stream, cell cycle related substrates by deactivating cyclin/CDKinase complexes. A number of genes in these mitogenic pathways, like ras and p53 are highly susceptible to mutation, where their modified structure and/or expression leads to a variety of cancers (Varmus and Lowell, 1994). Immature hematopoietic cells (stem cells) require initial stimulation by a specific growth factor (like stem cell factor) which promotes their controlled proliferation (Kaushansky and Karplus, 1993). Stem cell leukemias result from the inappropriate action of proteins at one or more potential points along the growth factor induced mitogenic or p53 shutdown pathways. It is my intention to use leukemia of the soft shelled clam as an accessible, easy to use model for investigating normal and malfunctioning mitogenic genes and gene products. Soft shelled clams (Mya arenaria) are bivalve molluscs which develop a fatal, diffuse blood tumor or leukemia. The leukemia is characterized by: a) large tumor cells which have a high nuclear:cytoplasmic ratio; b) an ability to transfer the disease when injected into normal non-affected clams and c) an invasion of solid tissue as the disease progresses. Unlike normal blood cells, leukemic cells undergo constant mitotic division (80% vs. equal to or less than 5%) and they are round and don't attach to culture dishes.The cause of the disease is unknown, although recent reports suggest the involvement of a retroviral vector. In order to develop this model system for use in studies of leukemic signal transduction mechanisms, I propose to examine the molecular basis for leukemia in clams by: (1) demonstrating the existence and differential functioning of a growth factor/tyrosine kinase receptor mitogenic pathway in normal and leukemic clam blood cells; (2) identifying genes involved in initiating and terminating cell division in normal blood cells that are modified in their structure and/or their expression in leukemic clam blood cells; (3) mass culturing clam leukemia cells and fully characterize them with microscopy and identified clam cell cycle genes or gene products; and (4) demonstrating whether normal cell division can be restored in leukemic blood cells by microinjection of proteins derived normal forms of the altered genes we identify. Because of the high degree of conservation in many of the genes and resulting proteins involved in the signal transduction cascades between mammals and clams, our studies may have substantial implications for understanding leukemia in mammals. Molecular data that we gather about leukemia in clams should point out mechanisms for diagnosis, treatment and prevention and should lead to an understanding of how the disease is transmitted and promoted.